The sexually dimorphic nucleus of the preoptic area (SDN-POA) is an anatomically distinct region in the rat hypothalamus that has 2-5 times greater neuronal mass in the male than in the female rat. The size, connectivity and chemical nature of the SND-POA develops during a critical period of the maturing rodent brain. This size differentiation of the SDN-POA is also dependent on pre- and perinatal exposure to steroid hormones. The present proposal will investigate whether or not this distinct morphological site differentiating the female and male anatomically also plays a neuroendocrine role in the sexual differentiation of the cycling female and non-cycling male-hypotalamic-pituitary axis and/or in the characteristic sexual behavioral patterns of the female rat. Electrophysiologic methods of single cell recording, antidromic activation, microiontophoresis of peptides and hormones, stereotaxic surgery and histological techniques will be used on Sprague-Dawley rats to investigate the following questions: (1) What is the relationship between the hormonal variations that occur during the estrous cycle and activity in the SDN-POA? This question involves the following considerations: (a) analysis of the electrical activity of SDN-POA neurons with hormonal changes over the estrous cycle; (b) determination of the sensitivity of SDN-POA neurons to luteinizing hormone-releasing hormone, estrogen and testosterone; (c) identification of non-antidromically and antidromically activated SDN-POA neurons via electrical stimulation of median eminence; and (d) comparison of electrophysiologic and neuropharmacologic characteristics of SDN-POA neurons in cycling female and non-cycling male rats. (2) What is the relationship between sexual receptivity and activity in the SDN-POA? This question involves the following considerations: (a) analysis of the electrical activity of SDN-POA neurons in ovariectomized and ovariectomized, estrogen-progesterone primed female rats; (b) determination of the chemical sensitivity of SDN-POA neurons to luteinizing hormone-releasing hormone, estrogen and testosterone; and (c) identification of non-antidromically and antidromically activated SDN-POA neurons via electrical stimulation of the ventromedial nucleus of the hypothalamus.